Abstract
Rechargeable aluminum batteries with multi-electron reaction have a high theoretical capacity for next generation of energy storage devices.However,the diffusion mechanism and intrinsic property of Al in-sertion into MnO2 are not clear.Hence,based on the first-principles calculations,key influencing factors of slow Al-ions diffusion are narrow pathways,unstable Al-O bonds and Mn3+type polaron have been identified by investigating four types of δ-MnO2(O3,O'3,P2 and T1).Although Al insert into δ-MnO2 leads to a decrease in the spacing of the Mn-Mn layer,P2 type MnO2 keeps the long(spacious pathways)and stable(2.007-2.030 Å)Al-O bonds resulting in the lower energy barrier of Al diffusion of 0.56 eV.By eliminated the influence of Mn3+(low concentration of Al insertion),the energy barrier of Al migration achieves 0.19 eV in P2 type,confirming the obviously effect of Mn3+polaron.On the contrary,although the T1 type MnO2 has the sluggish of Al-ions diffusion,the larger interlayer spacing of Mn-Mn layer,causing by H2O could assist Al-ions diffusion.Furthermore,it is worth to notice that the multilayer 8-MnO2 achieves multi-electron reaction of 3|e|.Considering the requirement of high energy density,the average voltage of P2(1.76 V)is not an obstacle for application as cathode in RABs.These discover sug-gest that layered MnO2 should keep more P2-type structure in the synthesis of materials and increase the interlayer spacing of Mn-Mn layer for providing technical support of RABs in large-scale energy storage.
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